The invention relates to improvements in sonde housings used in horizontal direction drilling to carry a radio transmitter that indicates the location and orientation of a drill head.
Horizontal direction drilling in geological formations is widely used to place product such as pipe, conduit or cable underground. Typically, the location and orientation of the drill head is monitored as it progresses through the earth so that corrections can be made to keep the actual path as close as possible to the desired path. The location and orientation of the drill head is signaled to the surface by a radio transmitter carried in a so-called sonde housing that is interposed in the drill string just behind the drill head. The sonde housing includes passages for fluid that is used in the drilling process and that advantageously cools the sonde housing so that heat from the drilling operation does not overheat the electronics of the transmitter.
Conventional prior art sonde housings have been fabricated by machining steel bars or tubes to provide a chamber for the transmitter and axial passages for the fluid. That fluid creates a jet at the drill face or provides hydraulic power for a directional drill motor and, as mentioned, cools the transmitter. Typically, the prior art sonde housings are relatively expensive because of the special machining operations that are performed to create the chamber and various passages through the full length of the housing. This cost is significant to a drilling company because the typical sonde housing has a limited life. The fluid that passes through the sonde housing is continuously recycled. Although it is filtered, fine sand particles remain in the fluid causing it to be highly abrasive. The fluid, because of its abrasiveness, wears away at the passages in the housing eventually destroying it. Another problem frequently encountered with known types of sonde housings is related to slots or other apertures formed in the housing wall that allow transmission of radio waves out of the metal housing. The slots are frequently filled with epoxy or other non-metallic material to exclude fluid from the chamber in which the transmitter is received. This material is prone to leak internally after a period of use with the result that the transmitter and its associated battery can become cemented in the chamber by fluid borne solids making it very difficult to remove the transmitter without harm.
The invention provides an improved sonde housing that can be economically manufactured and that has improved performance both in resistance to wear and resistance to internal leakage. Various internal parts, while being made of relatively inexpensive materials, are capable of an extended service life meeting or exceeding that of more expensive traditional materials. Still further, internal parts that are susceptible to wear by abrasion from the fluid being conducted through the housing are replaceable at relatively low cost. As disclosed, the sonde housing comprises an outer metal cylindrical shell or main body having tool joints at each end. The shell wall is slotted at circumferentially spaced locations for transmission of radio signals from the transmitter carried within the shell body. A cartridge assembly is positioned in the shell body to provide a sealed chamber for the transmitter, an annulus for conducting fluid through the housing and a sleeve to seal the radio transmission slots in the shell wall and to protect the shell wall from abrasion from the circulating fluid. The main parts of the cartridge are formed of a suitable plastic so that they are extremely cost effective and, advantageously, are inherently transparent to the radio wave signals generated by the transmitter.
In the disclosed arrangement of the housing, the sleeve of the cartridge not only protects the shell body from abrasion, but also by sealing the radio signal emitting apertures in the shell wall, avoids the seal failure problems normally encountered in the prior art where the apertures are sealed with epoxy.